This invention relates to an apparatus and method for electrolytic treatment which comprises etching a metal web electrolytically using alternating waveform current continuously, and particularly, relates to an apparatus and method for electrolytic treatment suitable for electrolytic etching of aluminum web during manufacturing a support for planographic printing plate.
Electrolytic etching of a surface of aluminum, iron or the like is widely utilized, and alternating waveform current is generally used due to required quality and the improvement in reaction efficiency. For example, Japanese Patent KOKOKU 56-19280 discloses an electrolytic etching treatment which can provide excellent roughening as a support for an offset printing plate by using an alternating waveform current impressed so that the electric voltage at anode is made greater than the electric voltage at cathode in electrolytic roughening of an aluminum plate.
In general, electrolytic etching is carried out at a current density of 10 to 100 A/dm.sup.2 in an acidic electrolytic solution, such as 1 to 5% nitric acid or hydrochloric acid. Upon treating electrolytically an aluminum plate as the metal web, Al .fwdarw.Al.sup.3+ +3e dissolution reaction occurs at an anode period, and H.sup.+ +e.fwdarw.(1/2)H.sub.2 .uparw. hydrogen gas generation reaction and Al.sup.3+ +30H.fwdarw.Al(OH).sub.3 aluminum hydroxide smut formation reaction occur simultaneously at a cathode period on the aluminum plate.
These reactions occur alternately according to the frequency of power supply, and, in general, it is possible to obtain a required roughened surface by adjusting these fundamental reactions by controlling electrolytic conditions, such as the type and concentration of electrolytic solution, temperature conditions, current density and charged quantity of electricity.
Hereupon, in the case of using alternating waveform current, there are differences between the treatment initiating part by the dissolution reaction and the terminating part caused by the current cycle on entering a metal web into an electrolytic bath and on going out of the bath, and shadow unevenness (uneven treatment) occurs in the cross direction to the traveling direction of the metal web. The uneven treatment generates according to the frequency of a power supply. For example, in the case of a treating speed of 50 m/min and a frequency of power supply of 60 Hz, the uneven treatment generates at an interval (pitch) of 1.39 cm in the longitudinal direction of the metal web. In view of massproduction, a higher treating speed and a higher current density treating speed and a higher current density treatment are desired. In the case of high strength aluminum support containing manganese or the like which recently increases, there is a tendency to manufacture a high quality support for planographic printing plate by lowering frequency.
On the other hand, the degree of the above mentioned ununiformity according to the frequency of power supply becomes stronger by higher treating speed, higher current density or lower frequency due to its generation mechanism.
In this regard, Japanese Patent KOKAI 1-230800 discloses an electrolytic apparatus which can remove the uneven treatment by providing low current density zones at the front end and rear end of an electrode, and making the remaining portion a constant current density zone. Japanese Patent KOKAI 4-289200 discloses a roughening method using a power supply frequency of 1 to 30 Hz.
However, according to the conventional methods mentional above, the uneven treatment according to the frequency cannot be removed entirely in the case of high treating speed, high current density or variation of electrolytic solution conditions.